Many process systems of today include flows of different fluids, where the flow is created by a fan or pump. In many cases, the flow rate and other quantities related to the flow are of crucial importance or at least of interest for controlling the process. The measured flow and related quantities are typically used as input parameters in monitoring, measuring and/or controlling processes. There is thus a general request in many process systems involving fluid flows for measuring flows and pressures caused by different flow-creating means, i.e. essentially fans and pumps.
The straightforward approach used in prior art employs the use of direct measurements of flow-related quantities. In DE 198 51 795 A1, a system and a method for textile treatment are disclosed. A respective pressure sensor is introduced within a flow line of treatment fluid immediately before and after a pump. The pressure difference is used to control the operation of the pump in order to provide the right flow of treatment fluid in the process.
Hence, in such system and systems of a similar design, a number of sensors have to be introduced into the flow, which always exposes the sensors to wear and potential damage. Furthermore, sensors introduced in-line also increase the risk for changing or disturbing the actual flow in an unfavourable manner. Replacement or repair of such sensors will typically cause an operation stop of the process system, which often is very costly. Also the sensors themselves are relatively costly. There are thus a number of disadvantages with flow measurements according to prior art.
In the patent application AU 27168/95, a pump monitoring apparatus is disclosed. This apparatus monitors the pump motor in order to achieve a measure of the run time of the pump. By an a-priori knowledge of the expected pump capacity and pump efficiency, a throughput can be calculated by multiplying with the actual operation time ratio. This procedure is only applicable where capacity and efficiency are known in advance.
In U.S. Pat. No. 4,781,525, a flow measurement system is disclosed, where blood flow rate through a centrifugal pump is measured. A rotational speed is obtained by a rotational speed sensor. The sensor is directly measuring the speed of the motor shaft, mechanically, optically or in any other direct manner, as clearly indicated by a broken line in the disclosed figures. Furthermore, a parameter representative of the motor torque is obtained. The blood flow rate is computed as a function of the measured rotational speed and the obtained motor torque. A problem with the devices disclosed in U.S. Pat. No. 4,781,525 is that a separate rotational speed sensor has to be provided, which is both expensive and in many cases also difficult to provide for mechanical reasons.